The present invention relates to a key input circuit.
For example, an audio signal processing system of a one-box type audio apparatus can be constructed as shown in FIG. 1.
FIG. 1 corresponds to one channel of a stereo system. Audio signals from a CD player, a tuner, a tape deck, etc. are supplied, via input terminals 1A-1N, to a function switch 2, where a desired audio signal is selected.
The selected audio signal is supplied to a surround circuit 3, where it is subjected to a process of correcting or altering a reproduction sound field, such as a surround process. An output signal of the surround circuit 3 is subjected to a frequency characteristic adjustment in a tone circuit 4, and a resulting signal is supplied to a speaker 7 via an attenuator circuit 5 for volume adjustment and a power amplifier 6.
A control means, i.e., a microcomputer 8 for system control is connected to the circuits 3-5. Various manipulation keys 9 are connected to. the microcomputer 8. The characteristics of the circuit 3-5 are controlled or altered in accordance with manipulations of the keys 9, whereby a desired reproduction sound field is obtained.
Each key 9 is constituted of a non-lock type push switch. The number of switches depends on the number of adjustment items of the circuits 3-5 and other factors, and tens of switches may be provided in some cases. It is not appropriate to simply connect such a large number of switches to the microcomputer 8.
Therefore, the switches that constitute the keys 9 are connected, in matrix form, to an output port and an input port of the microcomputer 8, and dynamic scanning is performed to judge which of the keys 9 has been pushed.
Alternatively, the switches that constitute the keys 9 are connected to voltage sources, for instance, a voltage divider circuit. When a certain key 9 is pushed, a DC voltage is output whose magnitude is different for each key 9. After being A/D-converted, the output DC voltage is input to the microcomputer 8, which then judges which of the keys 9 has been pushed.
However, in the former case of using dynamic scanning, scan pulses may be mixed, as noise, into an audio signal line. They are easily mixed into an audio signal line particularly in the vicinity of high-impedance circuits such as the attenuator circuit 5. If scan-pulses are mixed into an audio signal line, as a matter of course the sound quality is deteriorated.
Further, since scan pulses are formed consecutively during a key input waiting period, scan pulses mixed into an audio signal line causes the speaker 7 to produce a continuous sound like an oscillation sound. Thus, reproduction sound causes an uncomfortable feeling.
A commonly employed countermeasure against the above-mentioned type of noise is a method of preventing scan pulses from mixing into an audio signal line by using a shield plate. However, this method increases the cost of the system. In addition, there may occur a case in which a sufficient shielding effect is not obtained, depending on the layout of parts and circuits on a printed wiring board and the arrangement of wiring patterns.
On the other hand, in the latter case of using A/D conversion, although there occurs no noise mixing, the use of an A/D converter results in a cost increase. Further, since the microcomputer 8 is required to incorporate an A/D converter, the degree of freedom in selecting a microcomputer is low.